Not really. Every time you masturbate, you are ejaculating the sperm you have accumulated and lowering the sperm available for fertilization.
However this is only temporary. your body immediately begins to produce more. On the up side, masturbation gets rid of accumulated old sperm which may find it difficult to fertilize the eggs.
No, you've got millions of those little guys that come out with each ejaculate, so no worries there.
No. Many scientists believe that masturbation can improve sperm quality. The optimum abstention period before trying to impregnate a woman is about two days. If it has been less than two days since your last ejaculation, your sperm count is apt to be lower than normal.
However, as your time since last ejaculation increases past two days, both your semen and your sperm get to be of lower quality. The fluids that make up semen get old inside the body, and this is why ejaculate produced after a long abstinence period is more yellow and less white. The sperm that come out after that long are also older and not in as good condition for conception. Thus, masturbating every two days (assuming you can't have intercourse at that interval) is good for your sperm and semen quality. However, you don't want to be maintaining your current masturbatory frequency of more than twice a day when you're trying to conceive.
Frequent masturbation may impact the volume of semen produced if you masturbate frequently, but will not cause low sperm count.
If you are trying to get pregnant, decrease the frequency of masturbation to increase the volume of semen, and thus increase the number of sperm to possibly fertilize your partner's eggs.
The things that can cause low sperm count are tight pants, tight underwear, long hours of bike riding, steroids. Low sperm count is caused by excessive heat in the scrotum. Heat kills and damages sperm.